1. Field of the Invention
The present invention relates to an improved commutator for a motor and a fuel pump unit using the same.
2. Related Art
As a fuel pump unit for an automotive vehicle, an electromagnetically operated-type fuel pump unit includes an electric motor and a motor-driven pump. In this motor, an armature wound with coils rotates when electric power is supplied from an electric power source to the armature by sliding contacts between brushes and contact parts of a commutator. This motor rotates an impeller of the pump which in turn sucks fuel from a fuel tank and supplies fuel to an internal combustion engine of the automotive vehicle.
Each contact part of the commutator is generally made of copper. Since the life of the brush will become shorter due to the sliding wear of the brush as the hardness of the brush is lower, it is proposed to improve anti-wear characteristics of the brush by mixing amorphous carbon having a high hardness into a carbon material. The copper contact parts are likely to react with, for instance, fuel which is oxidized or includes sulfur components, resulting in corrosion. In case the copper turns to electrically conductive copper sulfide, the electrically insulated contact parts are likely to short-circuit.
U.S. Pat. No. 5,175,463 proposes forming contact parts of a commutator with a carbon material for restricting reaction of contact parts with fuel. The contact part made of the carbon material is inferior to the contact part made of copper in both hardness and mechanical strength. When the contact part made of the carbon material slides on the brush with the mixed amorphous carbon, the wear of contact parts progresses faster resulting in shorter life of the commutator.
Provided that the contact part is made of artificial carbon which is harder than natural carbon, the life of the commutator will be lengthened. However, as artificial carbon is expensive, the manufacturing cost will become higher.